Tensioners have been widely used to maintain proper tension in, and suppress the vibration of, a timing chain which transmits rotation between a crankshaft sprocket and a camshaft sprocket in an engine. In a typical engine, a tensioner lever is pivotally mounted on the engine. The lever has a shoe which is in sliding contact with the slack side of the timing chain. A tensioner, which cooperates with the lever, has a protruding plunger which presses against the back side of the lever, thereby imparting tension to the chain.
FIGS. 7 and 8 show a conventional ratchet-type hydraulic tensioner 500, which includes a ratchet locking mechanism 530. This tensioner is disclosed in the specification of Japanese patent application No. 2000-013068. In the tensioner 500, a plunger 510 protrudes from a housing 520, and is biased in the protruding direction by a spring 560. The end of the plunger is movable forward and backward relative to a front surface of the tensioner. A ratchet pawl 531 is pivotally supported on the housing body 520 in a recessed groove 525 formed between side walls 524 of the housing 520. The pawl is biased by a compression spring 533. Retraction of the plunger 510 is blocked by engagement of the teeth 534 of the pawl with ratchet teeth 511 formed on the outer periphery of the plunger 510 on one side thereof.
The tensioner 500 includes a hydraulic valve mechanism 550 having a check valve 551 incorporating a check ball or the like. Oil is fed under pressure, by an oil pump (not shown) activated by the engine, through the valve mechanism 550, to a high pressure oil chamber 540 inside the plunger 510.
When an impact force is applied to the plunger by the tensioner lever, since the ratchet teeth are engaged with the pawl teeth 534, the plunger 510 retracts against the biasing force applied by the plunger spring 560, through a distance within the range permitted by the amount of backlash between the ratchet teeth 51l and the pawl teeth 534. The initial retracting movement of the plunger causes the hydraulic valve mechanism 550 to close. As the plunger continues to move in the retracting direction while the valve mechanism is closed, oil in the high-pressure oil chamber 540 leaks out between the outer peripheral surface of the plunger 510 and the inner peripheral surface of the plunger-receiving hole 521. Because of the damping effect of this leakage of oil, the plunger absorbs the impact imparted by the tensioner lever.
When rate at which oil is supplied by an oil pump to the tensioner 500 increases rapidly, excessive oil is fed through the hydraulic valve mechanism 550 to the high-pressure oil chamber 540 in the plunger 510. The excessive oil will cause the plunger 510 to protrude toward the tensioner lever farther than necessary.
Excessive protrusion of the plunger may occur, in particular, when a cold engine is started, because the oil supplied by the oil pump is cold and its viscosity is high. Thus, the oil pressure in the high-pressure oil chamber 540 within the plunger 510 is higher than usual, and, as a result the plunger 510, is pushed out toward the tensioner lever farther than necessary.
In the case of a conventional ratchet-type hydraulic tensioner, the ratchet mechanism prevents the plunger from returning to its normal position after it has protruded excessively from the tensioner housing. As a result, the plunger 510 presses on the tensioner lever, which in turn exerts excessive pressure on the timing chain, holding the chain under an abnormally increased tension. The excessive tension in the chain generates a noise, referred to as a “whistling sound”, and also causes premature breakage of the chain.
Another problem encountered in the operation of the conventional ratchet-type hydraulic tensioner is that the supply of oil to the tensioner stops simultaneously with the stopping of the engine. The amount of oil in the oil passage leading to the high pressure chamber inside the plunger gradually decreases and the oil in the oil passage becomes mixed with air. The presence of air in the oil passage causes the behavior of the plunger to become unstable when the engine is re-started, and causes a troublesome delay in the reestablishment of appropriate chain tension.
An object of the invention is to solve the above-described problems encountered in the use of conventional tensioners. It is also an object of the invention to provide a ratchet-type hydraulic tensioner in which, when excessive oil is supplied from an external oil pressure source, the excess supply of oil to the plunger is suppressed, and abnormal protrusion of the plunger is prevented, so that the whistling sound is reduced. Still another object is to establish proper chain tension rapidly upon starting of the engine, and to ensure smooth travel of the chain so that damage to components of the tensioner can be significantly reduced.